1. Field of the Invention
The present invention relates to a phase shift full bridge converter, and more particularly, to a phase shift full bridge converter which is capable of reducing current stress by removing an output inductor and adding an auxiliary circuit unit to a secondary side of a transformer.
2. Description of the Related Art
Recently, a power supply circuit supplies power from a general-use input line (85 Vac˜265 Vac) through a power factor correction circuit to a DC/DC voltage terminal, and the power is transformed into a desired power at the DC/DC voltage terminal.
The power factor correction circuit is used in order to meet harmonics regulation that is recently reinforced internationally. The power factor correction circuit mainly employs a boost converter. Due to characteristics of the boost converter, its output is always higher than its input. Therefore, the output of the power correction circuit, that is, the input of the DC/DC voltage terminal, becomes 385 Vdc˜415 Vdc. In view of the DC/DC voltage terminal, a specification of a high input voltage is necessarily required when the power factor correction circuit is used.
A recent trend is to require an output specification of a low voltage and a high current as the output of the DC/DC voltage terminal. As one method for reducing power consumption of communication devices and processors, low-voltage processors are widely used. In addition, for user's convenience, functions of processors increase and thus their power consumption also increase. Therefore, most DC/DC voltage circuits have an output specification of a low voltage and a high current.
Recently, the main considerations of the DC/DC voltage terminal are a voltage stress at a primary side and a current stress at a secondary side as the specifications of a high input voltage, a low output voltage, and a high output current. Furthermore, in order for high power density, the power supply should have a simple structure and a small volume. To this end, an output filter and an input filter are made in small size by increasing a switching frequency. However, as the switching frequency increases, a total efficiency is reduced due to loss generated in the switching. Therefore, there is required a zero voltage switching that can reduce the switching loss. Moreover, conduction loss should be reduced by implementing a rectification terminal of an output unit with a synchronous rectifier instead of a diode. A phase shift full bridge converter is mainly used for meeting the above-described requirements.
In the conventional phase shift full bridge converter, when a duty is small, free wheeling current exists and a zero voltage switching region of a lagging leg switch is narrow. In addition, since an output inductor exists at a position where a high load current flows, it occupies a large volume. Moreover, in magnetic devices, power loss is caused by core loss and conduction loss. Therefore, in the power supply specifications such as a high input voltage, a low output voltage, and a high output current, the phase shift full bridge converter is suitable, but a large-volume output inductor should be used for the specification of the high output current.
To solve the above program, a phase shift full bridge converter having no output inductor was proposed. However, in this case, a leakage inductor of a transformer replaces the role of the output inductor. Thus, since a secondary current operates as a discontinuous conduction mode, there is a problem that increases current stress of a synchronous rectifier of a secondary side in order to supply a high load current.